Bumping Sound

I have a 97 Sable that I has a bumping sound occuring in the front left engine area. At first it sounded like it was coming from the front left wheel area. After inspection of what seemed lik every part of the front end that was going bad, parts were replaced. These parts were really going bad so it had to be the cause of the problem. The noise was gone, for about 2 weeks then it started again. I took it back to the mechanic and he said that the only thing left that it could be causing the noise was the strut. I didn't buy that idea. The noise seemed to only occurr when I was driving not idling sitting still. Howover over the weekend after changing the plugs, I cranked the car. I pressed the gas pedal and as the RPMs began to lower the noise occured. Everytime I pressed the gas, the noise would happen when the engine roar was gongi down. So, I know that it's not the strut, but what is it? It sounds like a combination of a bump and suck. From the cabin the sound comes from the passenger side of the engine. When riding in the passenger seat, you can feel a slight bump. Any ideas? HELP!!

Mike

After further investigating, I discovered that the noise is coming from the compressor clutch as it engages. It was low in coolant so I brought it up to the correct level, but the noise is still pretty loud and I can still feel it as a bump on the passenger side floor board area. Almost every time I revved up the engine the bump occurred when the RPMs were falling. It was as if the throttle controlled when the clutch would engage and disengage. Obviously the compressor is going out. Anybody ever experience anything like this? Any help would be great.
Mike